The iodothyronine derivatives 3,5,3'-triiodothyrionine (T3) and thyroxine (T4,3,5,3',5'-tetraiodothyronine) are important compounds for metabolic regulation. Thyromimetic activity is also found in other substituted thyronines. Previous research on structure-activity relationships has determined that the preferred conformation of T3, for example, has the 3'-I in the distal location, whereas, recent X-ray crystallographic investigations of T3, and a T3 analog have found a proximal conformation in the solid state. The objectives of this research are (1) to develop general methods based on the NMR-lathanide shift reagent (LSR) technique for the determination of molecular conformations in solution; (2) to apply these methods to the analysis of the solution conformations of several thyroid hormone analogs. The LSR's are paramagnetic La(III) chelates which form labile solution complexes with electronegative substrate atoms, and which cause the proton resonances in the substrate NMR spectrum to undergo upfield or downfield shifts. The shift magnitudes, which are related to proton-La distance and angle factors, can be used to determine various conformational features in the substrate. The method to be used in this work involves attaching the substrate, for example a diphenyl ether, to a rigid bicyclic framework containing an amide function. The LSR-amide geometry can be accurately determined. The conformation of the diphenyl ether substituent can then be ascertained using the known LSR-amide geometry.